Casteels has become Hoffenheim’s No. 1 keeper and could also be included in Belgium’s World Cup squad.
Casteels new deal will keep him at Hoffenheim until 2017 while Grahl’s new contract will keep him with the German side until the summer of 2016.
A Hoffenheim statement read: “1899 has signed contract extensions with Koen Casteels and Jens Grahl ahead of time.
“Casteels will stay at TSG until 2017, his contract would have been up in 2015. Grahl, whose contract would have been up next year, has signed for two more years until June 2016.”